This invention relates to electronic ballasts for operating discharge lamps such as fluorescent lamps, and in particular to such ballasts having a minimum number of active components.
Most magnetically coupled self-oscillating inverters are manufactured in large quantities for sale in a highly competitive market. Half-bridge inverters are widely used because they have a relatively low parts count. Such inverters may be classified into two groups: those using a current transformer having a saturable core, generally together with power BJT's (bipolar junction transistors); and those using a current transformer having a linear core, generally together with MOSFETs (metal oxide semiconductor field effect transistors). As those of ordinary skill will recognize, in this context a linear core is one in which operation is over a region having a curved B-H characteristic, rather than a sharp B-H characteristic; that is, at all times the flux level is such that a significant increase in magnetizing current will be accompanied by a significant increase in flux level.
U.S. Pat. No. 5,313,142 discloses such a ballast circuit having a saturable core inductor having secondary windings connected to the control electrodes of the switching transistors. This inductor is in a series connected load circuit which includes another inductor and a lamp, the lamp having series and parallel capacitors. Self oscillation occurs when current through the saturable core inductor rises to a level which saturates the core such that voltage in the secondary windings drops and the switching transistor, which was turned on, is turned off. This load circuit is connected between the output node of the inverter and a node between two feedback capacitors, one to the DC ground bus and one to the AC side of a rectifier circuit, which may be either a voltage doubler type or a full wave bridge. The other AC-side terminal is connected to a line inductor. High frequency power fed back through the feedback capacitor develops a high frequency voltage across the line inductor, thereby providing a high frequency boost to the input signals applied to the rectifier diodes.
The ballast disclosed in this patent operates a fluorescent lamp which requires significant warm-up time for filaments in the lamp. To prevent generating a damagingly high overvoltage on the DC bus, two zener diodes limit the feedback when the lamp is not lit. The load circuit, and a snubber capacitor in parallel with the load circuit, are connected between a half-bridge BJT inverter and a feedback node. High frequency current into that node is divided between a capacitor C4 to signal ground and the feedback capacitor C5. The values of capacitors C4 and C5 are chosen to cause the circuit to act like a high frequency boost converter, so as to improve the circuit power factor and reduce line harmonics. However, it is clear that the line current is still discontinuous.